(a) Field of the Invention
The present invention concerns an improvement in power supply circuits designed so that an AC voltage is applied to a transformer and the output at the secondary winding thereof is rectified and smoothed to derive a DC voltage.
(b) Description of the Prior Art
In the past, conventional power supply circuits of the abovesaid type is arranged so that, in case of, for example, a power supply circuit designed to perform full wave rectification, the charge current for the smoothing capacitor on the secondary side of the transformer flows concentrically in one time for a half cycle of the input voltage. Accordingly, the input current will have a pulse-like waveshape. For this reason, it should be understood that, in case the load is very light or in case it is absent, the peak value of the input current is low, but in case the load is heavy, the peak value of the input current becomes very large, which leads to the following inconveniences.
(1) Because the peak value of the input current is large, copper loss which is determined by I.sup.2 .multidot.R (wherein: I represents the value of current; and R represents the resistance value of the winding of the transformer) will become large, with the result that the efficiency of the circuit becomes poor.
(2) Since the input current is a pulse-like one having a large peak value, the rate of variation of current dI/dt is large, and the radiating noises become large, and furthermore because of the abundance of higher harmonic components contained in the input current itself, noises are generated. These noises may be removed to some extent by the inclusion of a shield or a filter, but there is a limit to doing so.
In addition to the above-mentioned known type of power supply system, there has been proposed a power supply circuit which is of the type called "switching regulator". This system is arranged to be operative so that an AC voltage is converted once to a DC voltage which is then converted to a pulse signal by a switching circuit using a high frequency, and this pulse signal is transformed into a required voltage by a transformer, and the resulting voltage is rectified and smoothed to thereby obtain a desired DC voltage. In this latter power supply system also, the current flowing in the current-rectifying and smoothing circuit on the primary side becomes a squeezed pulse-like narrow current as in the preceding prior art power supply circuit, and involves the problem of noise generation as in the preceding case. In the known power supply system just described above, the loss of power is produced mainly in the current-rectifying elements, and therefore this power supply system not only is disadvantageous in respect of efficiency, but also the generation of heat in the rectifying elements provides a problem. Accordingly, there has been the need to use a rectifying circuit having a large permissible current capacity. Furthermore, the degree of occupation of volume by the rectifying elements and the smoothing capacitor on the primary side in the known power supply system of this type amounts to about 1/2 to 1/3 of the whole system. Thus, these constituent components of the system contribute to an increase in the size of the whole power supply circuit, and thus to an increase in the manufacturing cost.
Also, with the aim to prevent the loss due to the exciting current of a transformer in the known power supply circuit which is designed to apply an AC voltage to the transformer so that the output on the secondary side thereof is rectified and smoothed to derive a DC voltage, and also to prevent an increase in the size of the transformer which is resulted from the need of said exciting current, there has been proposed a power supply circuit intended to perform switching-control, at the primary side of the transformer, of the AC voltage which is applied to the primary side of this transformer. For this matter, reference should be made to, for example, the copending U.S. Pat. application Ser. No. 129,642, U.S. Pat. No. 4,051,425, U.S. Pat. No. 3,466,527, U.S. Pat. No. 3,723,849 and U.S. Pat. No. 3,506,905. However, in these known power supply circuits, it should be understood that, although the problems concerning the efficiency and the increase in the size of the system resulting from an exciting current may be avoided, there still are entailed by such problems as the decrease in efficiency and the generation of noises due to the concentration of current as in the case of the abovesaid prior art power supply circuits.